1. Field of the Invention
The invention relates to a system and a process for reducing the water content of pulverized solid carboniferous materials and/or sludges, especially raw lignite. The water content is typically bound by capillarity in the fiber cells of the input material to be dewatered and is removed through the effects of thermal energy and pressure. The thermal energy consists of superheated water vapor and mechanical energy, both of which are supplied and exerted as surface pressure on the input material.
2. Description of the Related Art
A process and a device for dewatering carboniferous materials are disclosed in DE-PS 359 440, DE-PS 334 903 and DE-PS 339 034. These references describe a process and a device for dewatering peat and similar materials. The material to be dewatered is prepressed in thin vertical layers in circular cylindrical shafts, and after removal of the pressure, is exposed without pressure to the effects of high-pressure steam before undergoing a final pressing. Of special importance is the stage in the process during which the prepressed material is exposed to steam. Before the prepressed material which is contained in the circular cylindrical shaft and bounded by a ring-shaped pressure piston, is exposed to steam, the withdrawal of the piston creates so much space that the material can be stretched within its circular shaft, thus permitting the pressed cake to be broken up by lateral effect of the steam. Because the pressed cake breaks up, the high-pressure steam supplied in any given stage of the process can easily find its way through the input material and freely press away the loosened material so that channels can be formed through which large quantities of the steam pour with only limited effect on the material without purposeful condensation of the steam on all sides of the surface of the partially pulverized input material. By prepressing the material while it is in a cold state, the water, which can be squeezed out while cold, is extracted from the material. In peat, this water is present mainly as large quantities of surface water. Preheating of the input material with the large quantities of water which is not colloidally bound would be completely uneconomical in terms of energy and processing technology.
On the other hand, lignite contains only water which is colloidally bound, that is, water which is bound in the fiber cells by capillarity. Lignite has a water content of up to approximately 60 percent by weight. When this lignite is burnt in power plants, either a considerable proportion of the lignite input must be expended directly, or an adequate quantity of heat from the combustion gases must be used to vaporize the water. This proportion can be up to 22 percent by weight depending on the water content. This loss of energy can be reduced only if the water content of the raw lignite is reduced, before combustion, in an efficient drying or dewatering process.
The process, described in DE-PS 359 440, for releasing free surface water by prior dewatering under pressure is unnecessary with raw lignite. An additional disadvantage of such a process is the absence of control over steaming without pressure, and as a result, an adequate dewatering does not occur during the final pressing.
The devices in DE-PS 334 903 and DE-PS 339 034 for executing the process described in DE-PS 359 440 are, with regard to supplying of input material and emptying out of the dewatered and pressed input material, completely unsuitable for continuous throughput of large quantities, which is required for example for a power plant, and are therefore uneconomical. Steaming of the input material while it is being lightly precompressed within a range of steam pressure of 5 bar to 8 bar, for example, for uniform flowthrough of the pulverized raw lignite would not be possible in these devices, because the circular piston is not sealed off at the porous side walls and the tangential stretching due to the internal pressure creates an unacceptable gap between the circular piston and the inner walls of the cylinder. As a consequence, a substantial portion of the steam is lost and dewatering of only a limited amount is possible. This means that the use of such a variant of the process would be uneconomical with these devices.
Difficulties with using known processes for reducing the water content of lignite in large power plants are due to the fact that, as a consequence of the necessarily high throughput of lignite, the cost of equipment becomes very high when, for example, autoclaves are used in accordance with the Fleissner process with expensive pressure sluices, fans, and high pressure pumps. The use of this process for thermal dewatering has so far not produced any commercial successes, although the specific consumption of energy is lower in comparison with thermal drying. In order to ensure that a power plant will have a throughput of large quantities of input material to be dewatered, it is necessary to use large-surface filter presses with the greatest possible piling heights, for example about 500 mm, of the input material which is sprinkled in beds. This is also true for continuously operating double belt presses, which are disclosed in DE-PS 472 419.
With regard to the great piling heights, the use of a pressing system open at the sides is unsuitable when the compression ratio of the granulated raw lignite to the dewatered pressed lignite is 3:1 and the piling angle is approximately 32.degree., because of the large losses occurring at the edges. This is especially true during the steam supply segment of the process. This becomes still more critical for solid carboniferous materials with a more or less colloidally bound water content exceeding 65 percent by weight as, for example, in the case of plastically flowing sludges with a water content of approximately 75 percent by weight.
Sidewalls and bulkheads have been arranged as disclosed in DE-PS 472 419 for dewatering raw peat, in an attempt to stabilize the plastic flow consistency of the bulk flow inside the press by means of swinging vertical plates. However, the system in accordance with this reference does not provide for a controlled supply of steam into the total body of pressed material and its design is not suitable for this purpose.